I typically buy grit at the feed store, and I believe it is crushed granite, and the other is oyster shell for their calcium. I am a general contractor though, and have access to a lot of other natural stone from tear outs. I was wondering if you can give chickens crushed marble? or crushed travertine? I know I can give them limestone for calcium, and travertine is actually a form of limestone, so would this work the same way? And is granite the only kind of rock you can give them as an insoluble grit? or would any other forms of stone work? It's not really a big deal, I was just wondering because it seems like a shame to just always haul all this nice stone to the dump to get rid of it. I would worry about stone tiles since people use chemicals to mop their floors, but stone from walls or a mantle that I would consider clean etc... seems like a waste to just dump it. I searched and couldn't find info on other forms of grit other than granite to help them digest their food, and oyster shell or limestone for the calcium soluble grit, and thought it would be a good idea to find out.